The Line in the Sans
by KatieMarie999
Summary: After years of pranks, Papyrus has finally had enough and enlists Toriel to help him get the ultimate revenge on his brother. But no one expected Toriel to have such a mischievous side and before they know it, a full out prank war has been declared. Now no one is safe unless they ally with either Sans and Undyne or Papyrus and Toriel. Interactive fic! No pairings. Fem!Frisk
1. The Last Straw

_This idea has been knocking around in my head for a while. I posted it on an instagram chat and people seemed to like it. I want this story to be interactive so please include your ideas in reviews and I'll base chapters on them! I'll expand on that at the end of this introductory chapter._

 _Shipping wars are too rampant in this fandom so, aside from specific ship stories, I'm going to keep everyone platonic. Yes, I know this means retconning Alphyne but I don't want to bog it down with romance when this is meant as a comedy. I wanted to make this all inclusive and I know of too many opposing ships for it to work unless I just eliminate them entirely. There will be no ship bashing here, even of ones I personally dislike._

 _Frisk is a girl for this fic mainly because I don't like the "they" pronoun in the context of a fanfic (worked well in the game but it makes it a little confusing in written stories) and I always kind of imagined her as a girl._

 _Now that's out of the way, I hope you enjoy this silly little fic and please don't hesitate to make suggestions!_

* * *

 **Chapter One: The Last Straw**

* * *

The Great Papyrus had a reputation to uphold. True, surface life was a slightly bigger adjustment than he had expected. Sure the driving test had been a heck of a lot harder than he'd anticipated (why did humans insist on learning "parallel parking" anyway?). But he liked to think that he was adjusting just as well as everyone else.

Except Sans of course.

Sans knew the ropes of this new world. He slipped up a bit here and there but for all intents and purposes, it was like he'd lived with humans all his life. Papyrus rather thought it was like Sans was just a really smart guy who didn't live up to his potential. Why waste all that brainpower snoozing at illegal hot dog stands when he could have... could have...

Well, Papyrus didn't know what Sans could have done. Something smart and meaningful, he supposed.

But it was nice to know that some things never changed. He still met with Undyne for training sessions. He still lived in a flat with Sans (more out of habit than anything). He still created overly complicated puzzles whenever he could (though the crossing guard had been quite unpleasant when Papyrus had so cleverly laid out Alphys's tile puzzle on top of the crosswalk to give her an extra challenge).

Unfortunately, Sans was one of the things that never changed. And while this was a small comfort to Papyrus, it was also a source of irritation.

Not too long after they got to the surface, the weather had turned cool and Frisk started chatting about dressing up for a strange sort of holiday involving candy and pumpkins. Sans had taken great interest, helping the child make a costume of the strange, blonde singing lady in that weird snow movie they'd watched recently (Papyrus had taken a liking to the talking snowman but the rest of the film was... strange). On this day, Papyrus had been cooking in the kitchen and was about to serve them his new and Toriel approved spaghetti making technique when he opened the door to the living room. Instantly, a horribly loud sound filled the room and Papyrus nearly dropped the platter of spaghetti all over himself. Raucous laughter filled the room from Sans, Frisk, and Undyne.

"SANS!" he bellowed, whipping around to find an air horn duct taped to the wall the door had hit.

"Ahh come on, Papyrus." Sans shrugged, giving his brother a toothy grin. "There's no need to be _hornery_!"

"UGH!" Papyrus slammed the platter of spaghetti on the table, jumping out of the way of the sauce splatters before they harmed his precious uniform. "I give up!"

"Don't be like that, Papyrus." Undyne wheezed. "It was a harmless-"

"Harmless? HARMLESS?" Papyrus stamped his foot. "I nearly got sauce on my shirt! Do you know how long it would have taken to wash it out! What do I have to do to get some respect around here?"

"It's like you don't even have a funny bone." Sans shook his head almost sadly. "I guess only some of us can take a joke."

"A joke?" Papyrus swelled indignantly, which was quite a feat for him considering the fact that his body comprised of bones. "Freezing my spaghetti and serving it during my meeting with Asgore? Hiding 37 alarm clocks all over my room and set within minutes of each other? And I have to open every can in this house while pointing it away from me just in case a plastic snake comes jumping out! Well I've had enough!"

Papyrus ignored Sans, Frisk, and Undyne's calls for him to stay as he stomped out of the house, slamming the door behind him. He had barely progressed two feet before bumping into Toriel.

"Oh, I'm sorry." Toriel smiled warmly and took a step back to let Papyrus pass. "Are you all right, Papyrus?"

"I'm _fine_." Papyrus snapped in a voice so obviously upset that he didn't even convince himself.

"No you're not. Come on, calm down, let's talk about it." Toriel led him to a bench and placed her paw on his shoulder. Papyrus reluctantly sat down. "Now what's wrong?"

"It's nothing." Papyrus grumbled. But before Toriel could open her mouth to respond, he continued talking. "My brother is driving me insane!"

"Oh dear." Toriel smiled understandingly. "What did he do this time?"

"What _hasn't_ he done? Every day it's something different. If he's not unscrewing ketchup and mustard bottles enough for the top to come off, he's sneaking into my room and writing stupid additions to my to-do lists!"

"He's getting out of control?"

"He passed out of control years ago! Now he's unbearable! And I've had enough! There's no getting back at him! He's too good!" Papyrus groaned. "I'm tired of living this way."

Toriel was silent for a moment. "You know..." she said slowly. "You could ask for help."

"From whom? Undyne is always collaborating with Sans in these obnoxious endeavors!"

"No, silly." Toriel laughed. "From me!"

"From... from you?" Papyrus was caught off guard by this unexpected response. "You would help me get my revenge against Sans and Undyne?"

"It sounds like fun." Toriel said cheerfully. "Been a long time since I've had a partner in crime."

"Yes but you're so... so..."

"Yes?"

"Umm... mature!" Papyrus said hurriedly.

"I did have a life _before_ everything happened, you know." Toriel reminded him. "And this way I can make sure your plans aren't too dangerous. We don't want anyone to get hurt."

"Perhaps..." Papyrus thought about this for a moment. "I think it could work. Sans would never expect _you_ to be the mastermind behind it all! It's brilliant!"

"Well sometimes you do have to fight fire with fire."

"Wait..." Papyrus narrowed his eye sockets. "Are you saying I should start cooking the way Undyne taught me again?"

"No!" Toriel shook her head vehemently. "All right, I've already got an idea." Toriel donned a mischievous expression and explained her plan to Papyrus in a low voice. When she was finished talking, Papyrus looked giddy.

"Yes! That will teach him to stop these ridiculous pranks!" he exclaimed (though not too loudly or Sans and Undyne would hear).

"And if not, you can come back to me for ideas." Toriel assured him. "But I do have to go. Frisk has a dentist appointment in about a half hour and I don't want to be late."

"Of course." Papyrus stood up. "I will be taking your advice tomorrow morning."

"Let me know how it goes." Toriel said before pushing open the door to the flat, Papyrus at her heels.

"I've opted to forgive your nefarious pranking. This time." he said rather loudly before going into his room.

"Well that's a bit... suspicious." said Sans, staring at Papyrus's bedroom door. "What got into him?"

"I haven't the slightest idea." Toriel said calmly. "Come on, Frisk. Best not to be late."

As Toriel helped Frisk put on her coat, Undyne turned to Sans.

"This isn't over, is it?" she asked him.

"Oh no. I have a feeling this is just beginning." Sans rubbed his hands together. "And I can't wait."

* * *

 _I hope you liked this! Future updates will likely be a bit longer. Please give me some prank ideas because this has only just started and it's going to get crazy!_

 _Don't forget to review!  
_

 _~KateMarie999_


	2. A Declaration

_All of the suggestions I received were amazing! I think I'm going to use most of them and they inspired me to update this fic super fast. I haven't been this inspired to write in a very long time. It's that new fandom feeling. I missed it!_

* * *

 **Chapter Two: A Declaration**

* * *

Sans was fairly certain he would be able to figure out what kind of revenge Papyrus would dish out once Frisk and Undyne had left the house. But Papyrus had been uncharacteristically quiet, something that never boded well. If there was one thing Sans knew about his brother, it was that silence was highly suspicious. So when Papyrus went on a late night grocery run, Sans snuck into the kitchen to check out what he had bought.

More spaghetti sauce and noodles. So maybe there wasn't revenge coming his way.

Still, Sans felt a bit on edge and, as an extra precaution, locked his room up tight. By the next morning, he was fairly sure he was overreacting. Papyrus wasn't a prankster at heart. He was far too obvious. And forgiving.

As usual, Papyrus was up quite early, newspapers spread out in front of him. He didn't look up when Sans walked in, yawning and stretching.

"I assume you had a nice nap." he said shortly, turning the page.

"Pretty good." said Sans.

He opened the fridge a tiny smidgen and peered inside. There was nothing unusual in there. No bucket of water over his head that would drop on him were he to open the door any more. This was promising.

"So," he said, taking out a bottle of ketchup from the door of the fridge, "no hard feelings about last night?"

"Water under the bridge." Papyrus shrugged. "You know I can't stay mad at you."

"That's one of the many things I like about you, bro. Your forgiving nature." Sans said with a grin.

He took a huge swig of ketchup and proceeded to spit it out. His mouth suddenly felt like it was on fire.

"What the-" he popped off the cap and looked into the bottle. "What's _in_ this?"

"Oh, you like it?" Papyrus reached into his pocket and drew out a little bottle. "I thought my extra ingredient would add a bit of flavor."

Tears streamed from Sans's eyes. He threw open the refrigerator door and grabbed a half empty carton of milk, opening it and taking a huge swig. This, too, he spat out.

"It's sour!" he bolted to the sink and stuck his head under it, pouring water into his open mouth. "Why would you-"

"I've told you not to put the milk back in the fridge when you should be throwing it away instead." Papyrus said nonchalantly. "Serves you right."

He tossed the bottle over to his brother, whose head was now completely soaked. Sans looked at the little label.

"Ghost pepper hot sauce?" he wheezed.

"Well the man at the store was _very_ helpful and said it was the hottest they had." Papyrus leaned on the table a bit, clearly enjoying the spectacle. "That should teach you not to mess with me, The Great Papyrus! Nyeh heh heh!"

"You, sir, are playing a _very_ dangerous game." Sans withdrew his head from the sink, eyes still watering. "There will be consequences!"

"Well at this point, it's nothing I haven't dealt with before already." Papyrus chortled. "Not as fun from the other side, is it?"

"You think I've used the full extent of my abilities?" Sans's blue eye gleamed. "You have no idea."

Papyrus turned back to his newspaper, not concerned about the mess Sans had made in the kitchen. He would clean it up later. Meanwhile Sans dug in the fridge for a fresh carton of milk. Underneath the table, Papyrus texted Toriel:

"SUCCESS!"

As Sans poured milk all over his mouth and, indeed, his face, Toriel quickly replied:

"It is not over yet."

* * *

After a quick, cold shower, Sans opened his closet and reached in for his hoodie. His hand met midair.

"Papyrus!" he bellowed. "What did you do to my hoodie?"

No answer. Evidently, Papyrus had left the house. Grumbling to himself, he opened his drawer to get out something else to wear. Much to his irritation, a different sight met his eyes.

"Ohhh he is going to have a _bad_ time." Sans muttered angrily as he pulled on the only shirt available. "A _very_ bad time!"

Twenty minutes later, Sans secured his bicycle and entered Grillby's. Everyone inside was laughing about something, though he couldn't quite see what.

"I'm going to need something extra strong to-" Sans began when he finally caught sight of the group sitting up at the bar.

It was a very strange sight. There was Papyrus, grinning triumphantly. Beside him was Toriel and... _himself_. Someone just about his size in _his_ hoodie.

He only had a split second to stare before the figure in his clothes turned around and grinned at him. It was Frisk, looking very amused.

"Nice shirt." she said with a giggle.

Sans looked down at the blue and pink striped shirt he was forced to wear. While it looked quite cute on Frisk, it looked ridiculous on him.

"Ha. Ha." Sans opened his arms wide, showing off the shirt. "Yeah, go ahead. Laugh at me. It's what I'd do!"

"Come. Sit." Frisk patted a seat next to her.

Sans gave her a suspicious glare before walking over to sit on the stool. A loud, rude noise drowned out the nearby conversation and several people around them began to laugh harder.

"Careful, kid. Sometimes people put whoopie cushions on the seats." Frisk said, a mischievous glint in her eye.

"Oh very funny, kid. Let me guess, my brother put you up to this." he glanced sideways at Papyrus.

"Actually," Frisk turned to Toriel, "it was Mom's idea."

Sans stared at Toriel with wide eye sockets. "Oh _really_?"

"Well you can't be the only one to have all the fun." Toriel shrugged.

"You're outnumbered, Sans." Papyrus chortled. "You might as well give up and leave your practical jokes in the past."

"You just wait, Pap." Sans said, his mind whirring with ideas for revenge. "All right, Grillby, I'll have an order of fries."

"It's on me." Frisk said in an uncanny impression of Sans. "Just put it on my tab."

"All right, that's pretty good." Sans admitted. "Now give me back my hoodie."

With a shrug, Frisk handed the hoodie to Sans, who immediately put it on, mostly to hide the ludicrous striped shirt he was forced to wear. When he put his hands in his pockets, a weird squeaking noise emanated from it. He extracted two rubber chickens.

"Heh heh heh, all right." he shook his head, smiling at the joke. "If that's the way you want to play it."

He heard Papyrus's triumphant laugh and decided that revenge would be sweet.

* * *

After a long day, Papyrus liked to watch Mettaton's game show with a frozen treat. He liked Nice Cream a lot and always kept a hidden stash. He had left Grillby's hours ago and Sans still wasn't back.

The second Papyrus opened the freezer door, he heard a loud squelching noise and was propelled backward by something cold and white. Wiping the substance off his face, he saw an empty Gaster Blaster sticking out of the freezer, dripping with vanilla Nice Cream.

"Smile!" came a voice just outside the window.

Papyrus looked over to find Undyne filming the incident with her phone. Flinging Nice Cream off his hands, the skeleton narrowed his eye sockets.

"Fuhuhu!" Undyne cackled. "This is definitely going on youtube!"

Papyrus stared at the kitchen, its floor now covered in the Nice Cream. He would have taken another stupid prank as revenge. But this wasn't a stupid little prank. This took effort. This took an accomplice. As he tried to stand up, his feet slipping all over the mound of Nice Cream, he glared at the window, where he could just barely make out Undyne running away.

"This. Means. War."

* * *

 _And it begins! Special thanks to **deadlyswarm** , **AceLegends** , and **NRMania** for your ingenious prank ideas! Keep them coming and I'll keep the chapters coming!_

 _Don't forget to review!  
_

 _~KateMarie999_


End file.
